Figure 3.2-1Examples of global emission pathways for the period 2010–2050 with global CO2 emissions capped at 750 Gt during this period. At this level, there is a 67 % probability of achieving compliance with the 2 °C guard rail (Chapter 5). The figure shows variants of a global emissions trend with different peak years: 2011 (green), 2015 (blue) and 2020 (red). In order to achieve compliance with these curves, annual reduction rates of 3.7 % (green), 5.3 % (blue) or 9.0 % (red) would be required in the early 2030s (relative to 2008).

I added an asterisk to the title, a footnote of warnings. Here they are:

*Warning: several assumptions which may turn out to be invalid are embedded within the diagram. Errors are likely to be on the bad side.

The commonly talked about 2°C limit isn’t really a limit, rather it’s a two thirds probability. (Would you fly a plane with a one in three chance of crashing?)

The 2°C limit is a political, not scientifically based, obligation.

Global average warming of 2°C is disastrous for the climate over large areas of the world.

That much CO2 in the atmosphere spells disaster for marine ecosystems through ocean acidification.

The emission scenarios associated with 2°C limit are based on Global Circulation Models (GCMs) that have two significant limitations:

The Butterfly Effect, whereby outcomes are tightly dependant on initial conditions. These are known unknowns that can at least be dealt with statistically.

The Hawkmoth Effect, whereby outcomes are tightly dependant on the models’ accuracy in representing the real world. This is an unkown unknown, much more difficult to control.

Even supposing the 2°C had some valid basis, we are currently heading on a trajectory that overshoots it by a disastrously wide margin.

Last Thursday the EU leaders agreed to limit our greenhouse gas emissions, reducing them by at least 40% on 1990 rates, in order to meet our obligation to keep average global warming to 2°C, but as we can see, such a figure is pretty meaningless. For any rational use of the precautionary principle, we have no ‘carbon budget’ left. The area under the Ski-slope graph has shrunk to nothing if we are looking for a sensible probability of over 90% of avoiding disaster. For those low-lying parts of the world that will be affected by the already crossed threshold of polar ice melt tipping points, disaster is already locked it. The game we have to play is adaptation and maximising mitigation. Setting up carbon budgets to avoid 2°C warmings is just so much tilting at windmills.